[PATCH 2/2] mm: keep nid around during hot-remove

[Pasha Tatashin]

On Wed, Aug 7, 2024 at 7:50 AM David Hildenbrand <david at redhat.com> wrote:
>
> On 07.08.24 13:32, David Hildenbrand wrote:
> > On 07.08.24 00:14, Pasha Tatashin wrote:
> >> nid is needed during memory hot-remove in order to account the
> >> information about the memmap overhead that is being removed.
> >>
> >> In addition, we cannot use page_pgdat(pfn_to_page(pfn)) during
> >> hotremove after remove_pfn_range_from_zone().
> >>
> >> We also cannot determine nid from walking through memblocks after
> >> remove_memory_block_devices() is called.
> >>
> >> Therefore, pass nid down from the beginning of hotremove to where
> >> it is used for the accounting purposes.
> >
> > I was happy to finally remove that nid parameter for good in:
> >
> > commit 65a2aa5f482ed0c1b5afb9e6b0b9e0b16bb8b616
> > Author: David Hildenbrand <david at redhat.com>
> > Date:   Tue Sep 7 19:55:04 2021 -0700
> >
> >       mm/memory_hotplug: remove nid parameter from arch_remove_memory()
> >
> > To ask the real question: Do we really need this counter per-nid at all?
> >
> > Seems to over-complicate things.
>
> Case in point: I think the handling is wrong?
>
> Just because some memory belongs to a nid doesn't mean that the vmemmap
> was allocated from that nid?

I believe when we hot-add we use nid for the memory that is being
added to account vmemmap, and when we do hot-remove we also use nid of
the memory that is being removed. But, you are correct, this does not
guarantee that the actual vmemmap memory is being allocated or removed
from the given nid.

> Wouldn't we want to look at the actual nid the vmemmap page belongs to
> that we are removing?

I am now looking into converting this counter to be system wide, i.e.
vm_event, it is all done under hotplug lock, so there is no
contention.

Pasha